In some sporting activities, especially alpine skiing, lateral or shear forces on the knee joint between the upper and lower leg are often in excess of the stress that the knee can withstand. These stresses are not limited to accidents in which bones break, but are often cumulative in their effect, acting to deteriorate the ability of the knee joint to perform its inherent function.
The knee joint with its covering, the patella, is intended to flex substantially in a motion represented by a person rising to a full-standing position, with the back straight, and using only the legs to effect the standing posture. The knee accomplishes this through a physiological system of bone structure, muscles and tendons.
The knee joint pivots about a polycentric center of rotation. Tendons of the upper and lower leg meet at points proximate, above and below, the knee joint. These points of attachment may be considered to be a polycentric center of rotation about which the knee action takes place.
Muscles and tendons, as indicated, do not act upon a single point in the knee joint structure, but rather serve to apply tension along a variety of points which can be represented as constituting a polycentric set of pivot points. This physiological apparatus is, for the most part, dedicated to providing the greatest resiliency and strength in a motion bringing the knee straight up along the front of the torso, with the upper leg hinged to the hip joint.
There is comparatively little musculature or ability of the knee joint to bend or rotate in a direction lateral or sideways to the direction a person is facing. The knee pivots, for the most part, in the direction the feet are facing. Any sideways motion is achieved through the musculature rather than through the action of the knee joint.
For this reason, virtually all athletic brace apparatuses make allowance for the polycentric pivot of the knee, and virtually all strive to reinforce the leg in its most vulnerable position, that is, when excessive lateral forces are applied to the leg. It is the transmittal of these excessive forces away from the vulnerable knee joint that is the objective of the vast majority of these devices, to displace the forces away from the knee. The muscles, tendons and stronger parts of the upper and power leg must absorb the majority of these forces. By transmitting the force to another part of the leg, strain on the knee is thereby relieved.
Forces which are lateral to the normal action of the knee, may be encountered in patients suffering from multiple sclerosis, cerebral palsy, paraplegia, hemiplegia, neuropathies, traumatic lower extremity injuries, and birth defects. This wide variety of diseases and injuries give rise to a condition in which the patient's knee joints have even less resiliency than normal and therefore cannot function to sustain the weight of the individual through the normal action of the knee joint.
In all cases, those arising by certain exercises or those cases arising through a variety of causes indicate above, the individual needs assistance in maintaining balance through an apparatus that can transmit certain forces around the knee joint, thus relieving the knee joint of physical stresses beyond its limit of endurance.